Permselective membranes suitable for use in the desalination of aqueous solutions are the subject of numerous patents. Scala et al (U.S. Pat. No. 3,744,642) disclosed the method of creating a thin film permselective multilayer membrane composed of a microporous substrate onto which were deposited (at the interface between an aqueous layer and an organic layer), a polyamide, a polyester, or a polysulfonamide polymer. These were linear polymers created at or in the surface of the microporous support (aqueous phase filled with diol or diamines) by the direct reaction of diols or diamines with diacyl halides of the carboxyl or sulfonyl types contained in an organic phase such as ethylene dichloride or hexane. This initial composite membrane consisting of linear (but water immiscible) polyesters, polyamides, or polysulfonamides (reacted onto or near the surface of microporous polymers by interfacial polycondensation); were subsequently shown to de-salt saline water solutions with high flux rates due to the thinness of the chemically created barrier film. The microporous support served only to support this salt rejecting chemically created film called the "thin film" of a thin film composite membrane.
Cadotte (U.S. Pat. No. 4,277,344) discloses permselective multi-layer membranes in which there is a microporous polysulfone substrate layer and a juxtaposed polyamide layer made from an aromatic triacid halide such as trimesoyl chloride and the primary amine m or p phenylene diamine (or substituted m or p phenylene diamines). Here, thin films were created which were of the crosslinked type resulting from the direct interfacial condensation of trimesoyl chloride or mixtures of trimesoyl and phthaloyl chlorides from an organic phase, with an aqueous phase containing either m or p phenylene diamines or substituted m or p phenylene diamines.
Hara et al (U.S. Pat. No. 4,353,802) discloses semipermeable composite membranes in which the membrane material is crosslinked using polyfunctional aromatic acid halides. Kamiyama et al (U.S. Pat. No. 4,619,767) discloses permselective multi-layer membranes having (1) a microporous substrate layer (2) an ultra thin layer of a crosslinked polyvinyl alcohol/amino compound and (3) a porous inner layer of water insoluble polyvinyl alcohol located between the porous substrate and the ultra thin layer. Other patents disclosing the preparation and properties of thin film composites are U.S. Pat. Nos. 3,951,815; 4,005,012; 4,039,440; 4,872,984; 4,857,363; 4,853,122; 4,243,701; 4,259,183; 4,242,208; 4,761,234; 4,769,148; and 4,802,984.
Thin film composite (TFC) membranes appear to be the most efficient reverse osmosis (RO) membranes known today. Heretofore, these membranes have been most conveniently made by interfacial condensation polymerizations to yield ultra thin films of polyamides, at the interface of an aqueous phase layer and a non-aqueous or organic (solvent) phase layer. Since the two phases are substantially immiscible, a super thin film will be generated at the interface of the two liquid layers. Since the reaction, which draws reactants from both phases is diffusion controlled and self limiting, the method yields ultra thin films, typically 50-5000 .ANG. thick.
Heretofore, such films (TFC's) were made upon, or just beneath the surface of a supporting microporous support film into which has been absorbed an aqueous diamine (or polyamine) solution; by simply placing the imbibed aqueous amino microporous substrate in contact with an organic solution containing a triacyl halide, a diacyl halide or a mixture of tri and diacyl halides. Reaction at only the interface yields the desired thin film polyamide; which, upon creation, forms a barrier whence no diamine can further contact any polyacyl halide contained in the organic phase. The creation of the thin film shuts down any further condensation polymerization.
An object of this invention is to produce a reverse osmosis (RO) or ultrafiltration (UF) membrane for use in the separation of salts, gases, liquids, solvents, and in the case of UF, small to medium sized molecules from mixtures in solution. The membrane is produced by causing a polymer dispersed (colloidially) in an emulsion to come out of its micellar phase and precipitate in such a manner as to form a thin film or barrier on a porous support, typically a polysulfone porous support.
The customary method used to make thin film composite RO membranes is to react a solution of an amine with a solution of an acyl halide or isocyanate to form a polymer at the interface of the two solutions containing their respective reactants. Such reactions proceed at high rates and the resulting thin film polymer is of varied molecular weight, thickness, and pore uniformity. In addition, in these interfacial polymerizations, the acyl chlorides, isocyanates, etc., employed to achieve phase separation, are customarily dissolved in an organic solvent that is often flammable (i.e. hexane) or a listed "suspect carcinogen" (i.e. ethylene dichloride).
The techniques of the present invention produce a film via coagulation, flocculation or precipitation of the emulsion solids by use of selected salts (NaCl, KCl, NaOAc, etc.) whether added directly or created by neutralization of the acid or base stabilized emulsion system by counter parts from the precipitating solution, or by simple evaporation. These emulsion precipitating techniques can also be accomplished at the interface between the organic and aqueous phases by neutralization and salting out (as depicted in the following equation) whence the stabilized emulsion (latex) polymer which is usually absorbed within the microporous support is precipitated at the interface by the generated Na.sup.+ Cl.sup.-. ##STR1##
In other cases it may be desirable to use one of the solvents common to the usual production of TFC membranes. Even in these cases, however, the advantage of the present invention of having a preformed, tailor made polymer (not one of the random type often produced by the usual interfacial condensation polymerization) is considerable. In the case of vinyl and related polymers, it is historically known that the most desired polymers having the highest molecular weight and linearity are produced by micellar emulsion polymerization. Here again, by using this invention, we create a thin film of vinyl and related polymers, or copolymers, having in most cases the best in properties of uniformity, toughness, elongation, solvent resistance, color, etc.
The flocculation, coagulation, or precipitation of the polymer from the emulsion onto the porous support film may also be accomplished by gases or mists (HCl is an example). It may also be desirable to add certain ingredients such as sodium hydroxide to the emulsion system to facilitate the production of salts, such as sodium chloride or sodium acetate, when dipping into the coagulating solution.